The official site implies mystery surrounding Arkgain.

I still think the PT are like Pokemon, you need to capture them or make a friendship, pact or something (to get a summoning item?).
Harken has the special skill "Ultimate Gespenst Kick" with Gespenst.
Maybe having/equipping different PTs will unlock different special skills.

Nacht and Abend change color when they power up for attacks.
Guess that "Portal" is the Crossgate at Formido Heim?
Any clue in previous games on who opens Crossgates and stuff?

What I can decipher about the other characters:

Kaldia has been on waiting for 20 years?
"Suzaku Blade"? Then could there be 3 other andriods to fill Genbu, Seiryu and Byako?
KOS-MOS, Ashen and one more(maybe one that is one but is hidding it)?

Otone, leader of the Nanbu family's ninjas "Uragenbu".
Now she guards and manages Kagura Amahara's Crossgate that is at the extended island Ryukoutou.
Fights with some metal cutting string.

Katze, the captain of the super cat battletank that travels Dunepolis.
A beastman born in Elfe Tale. Seems to have a mysterious past.
Agile speedy fighter.

Shuten, leader of "Nugijou" the city of the Shikioni race (north-eastern of Kagura Amahara).
Suzuka Hime is born in Nugijou too.
There seems to be something with Suzuka and Shuten.
Very rough trans+guess- Suzuka is betrothed to Shuten, she doesn't like it and ran off to Nanbu family.
The Shikioni race seems to want to conquer Kagura Amahara.
Shuten fights with a Naginata and uses lightning and flame spells.

Ezel, nothing much revealed.
Leader of the Orchestra Army as stated before.
Uses some explosive skill "Max Axe".
(If he really pulls a Woodan then makes him the forth andriod)

Henne, not much also.
A hot headed winged race female belonging to the Orchestra Army.
Uses a beam sword "Schnabel Saber", specialize in aerial battles.

kyon, Orchesta Army member.
Uses a Senjitsu Cannon that fires magic bullets (tornado, snow storm, elemental stuff).
Seems to fight while on some Bronte Craft (hover craft/platform?)
Specialize in air battle.

Ann, a mermaid resident of underwater country Varnai Kanai.
Captain of the pirate ship "Silent Works".
She does her job as captain and leave the subordinates to Bonny.
Uses the "Bubble Cannon" and her tail to attack.

Bonny, half-fishman and vice-captain to Ann.
Uses a double barrel shotgun "Persuader".

Koma, a female catgirl merchant.
Very rough trans+guess- Sells stuff from the Crossgate.
Shop rooted or travelling merchant?

Shirou, shopkeeper of a shop in Kagura Amahara.
Buys excavated items from Harken and other hunters.
Very rough trans+guess- A friend of Harken.